1. Field of the Invention
The present invention relates to a magnetic field generator and an assembling method thereof. More specifically, the present invention relates to a magnetic field generator for MRI and an assembling method thereof.
2. Description of the Related Art
Japanese Utility Model Publication (of examined Application for opposition) No. 5-41530 discloses a technique for reducing magnetic flux leakage in a magnetic field generator for MRI. According to the technique, a plate yoke has a main surface and another surface facing away therefrom. Permanent magnet blocks are disposed in said another surface of the plate yoke. On the main surface, shielding magnets provided by eight ferrite magnets are disposed in an annular pattern, correspondingly to an outer circumference of the permanent magnet blocks, thereby reducing the magnetic flux leakage.
However, according to this related art, it is difficult to effectively reduce the magnetic flux leakage in a magnetic field generator for MRI which generates an intense magnetic field not smaller than 0.3 T. It is also difficult to effectively reduce the magnetic flux leakage in an open type magnetic field generator for MRI which generates such an intense magnetic field.
Further, according to the above related art, the shielding magnets are exposed on an outer side of the magnetic field generator, potentially attracting such a magnetic object as a tool and/or a chain, during assembling and/or transportation. In such an instance, the shielding magnets, which are sintered bodies, can be destroyed when the attracted object hits the shielding magnets. Another problem is that if the object is large, the object sometimes cannot be taken off by a human power once it is drawn onto the shielding magnets.
Further, a recent magnetic field generator for MRI has legs. The legs are magnetic, and are attached to the main surface of the plate yoke after the shielding magnet are fixed (bonded) onto the main surface of the plate yoke. During the attaching operation, the legs can be pulled by the shielding magnets, potentially causing a danger to workers.
These problems are conspicuous when utilizing sintered rare-earth magnets, which have strong magnetism, as the shielding magnets for a purpose of weight reduction of the magnets.
Further, recently, there is proposed and prevailing a magnetic field generator having a continuous open space as wide as not smaller than 150 degrees as viewed from the center of the magnetic field generating space, in a front portion of the generator. In such a generator, magnetic flux leakage is large in the open space, and the problem becomes more conspicuous if the plate yoke of the generator has its front portion sloped for reduced weight, or generates an intense magnetic field not smaller than 0.3 T. The magnetic flux leakage can be reduced to a certain extent by increasing thickness of the plate yoke at the front portion of the generator. However, this is impossible in the magnetic field generator having a continuous open space as wide as not smaller than 150 degrees, due to an instable support structure.
It is therefore a primary object of the present invention to provide a magnetic field generator capable of reducing the magnetic flux leakage i.e. unwanted magnetic field outside of the magnetic field generator, more effectively.
Another object of the present invention is to provide a magnetic field generator capable of preventing damage to the shielding magnets.
Still another object of the present invention is to provide a magnetic field generator and an assembling method thereof, that can be assembled safely.
According to an aspect of the present invention, there is provided a magnetic field generator comprising: a pair of plate yokes opposed to each other with a space in between; a permanent magnet disposed on a main surface of each plate yoke, the two main surfaces being opposed to each other; a shielding magnet provided on another main surface, in at least one of the plate yokes; and a spacer placed between the plate yoke and the shielding magnet.
According to this invention, by providing a spacer, the shielding magnet can be spaced off the plate yoke. Therefore, magnetic saturation of the plate yoke can be suppressed or eased, and therefore the magnetic flux leakage can be reduced without decreasing the magnetic field strength in the space between the pair of plate yokes.
Preferably, the spacer is made of magnetic material. Use of a magnetic member as the spacer enables to virtually thicken the yoke at a place prone to magnetic saturation. This decreases magnetic reluctance, thereby increasing magnetic field strength in the space without major increase in weight.
According to another aspect of the present invention, there is provided a magnetic field generator comprising: a pair of plate yokes opposed to each other with a space in between; not more than two supporting yokes magnetically connecting the plate yokes; a permanent magnet disposed on a main surface of each plate yoke, the two main surfaces being opposed to each other; and a first shielding magnet and a second shielding magnet provided respectively at a forward portion on an open side and at a rearward portion, of another main surface, in at least one of the plate yokes.
According to this invention, provision of the first shielding magnet in the forward portion on the open side of said another main surface of the plate yoke enables to reduce unwanted magnetic flux leakage from the front portion of the generator. Further, the second shielding magnet enables to reduce magnetic flux leakage at the rear portion of the generator.
According to another aspect of the present invention, there is provided a magnetic field generator comprising: a pair of plate yokes opposed to each other with a space in between; a permanent magnet disposed on a main surface of each plate yoke, the two main surfaces being opposed to each other; a shielding magnet provided on another main surface, in at least one of the plate yokes; and cover member provided over the shielding magnet.
According to this invention, the cover member protects the shielding magnet, enabling to prevent damage to the shielding magnet.
Preferably, the cover member is made of nonmagnetic material. This enables to reliably reduce the magnetic flux leakage without shorting the magnetic flux generated by the shielding magnet.
Further, preferably, a distance not smaller than 2 mm is provided between an outer surface of the cover member and a surface of the shielding magnet. With this arrangement, attraction exerted to a magnetic object (such as a tool) by the shielding magnet can be decreased. Therefore, even if the magnetic object is drawn to stick, it can be removed easily from the cover member.
According to still another aspect of the present invention, there is provided a magnetic field generator a comprising: a pair of plate yokes opposed to each other with a space in between; a permanent magnet disposed on a main surface of each plate yoke, the two main surfaces being opposed to each other; a shielding magnet provided on another main surface, in at least one of the plate yokes; and a nonmagnetic leg formed on said another main surface of the plate yoke on which the shielding magnet is provided.
According to this invention, since the legs are nonmagnetic, the legs are not drawn to the shielding magnet at a time of assembling operation, and therefore the workers are no longer exposed to the danger.
According to still another aspect of the present invention, there is provided a magnetic field generator comprising: a pair of plate yokes opposed to each other with a space in between; a permanent magnet disposed on a main surface of each plate yoke, the two main surfaces being opposed to each other; a supporting yoke magnetically connecting the plate yokes and providing a continuous open space having an opening angle not smaller than 150 degrees as viewed from a center of a uniform magnetic field space between the permanent magnets; and a shielding magnet provided correspondingly to the open space, on another main surface, in at least one of the plate yokes.
In an open type magnetic field generator such as the above, there is a large magnetic flux leakage on the open side of the generator. Therefore, it is effective to provide a shielding magnet at a location of the plate yoke corresponding to the open space.
Preferably, the plate yoke has a gradually decreasing thickness, with said another main surface of the plate yoke having a sloped surface, and the shielding magnet is provided on the sloped surface. Magnetic flux leakage increases if part of the plate yoke is cut off for weight reduction of the generator. Therefore, it is effective to provide a shielding magnet at a place where the plate yoke is thinned.
The present invention is suitable for a magnetic field generator which generates a magnetic field not smaller than 0.3 T in the uniform magnetic field space. Magnetic flux leakage is larger in a magnetic field generator which generates such an intense magnetic field. Therefore, it is effective to provide a shielding magnet.
Preferably, the shielding magnet is a sintered rare-earth magnet. By using the sintered rare-earth magnet, which has strong magnetism, as the shielding magnet, together with the spacer, magnet flux leakage can be reduced more effectively with a smaller amount of magnet, without causing magnetic saturation.
According to still another aspect of the present invention, there is provided a method of assembling a magnetic field generator having a plate yoke, the method comprising steps of: first mounting a shielding magnet on a main surface of a spacer, and then placing another main surface of the spacer on a main surface of the plate yoke.
According to this invention, the shielding magnet is attached in advance to the spacer, and the spacer as mounted with the shielding magnet is attached to the main surface of the plate yoke. Therefore, the shielding magnet can be installed easily to a desired location on the spacer, and the magnetic field generator can be assembled safely.